From beheadings to summary executions to amputations to crucifixions, the terrorist group has become the most feared organization in the Middle East. That fear, evidenced in fleeing Iraqi soldiers and 500,000 Mosul residents, has played a vital role in the group’s march toward Baghdad. In many cases, police and soldiers literally ran, shedding their uniforms as they went, abandoning large caches of weapons.

The commitment to shocking violence is at the heart of both ISIS’s recruitment and appeal. To radicalized Islamists across the world, there’s something enticing in ISIS. It has attracted at least 12,000 fighters — 3,000 from the West — since its inception several years ago. . . .

Islamist terrorism, [terrorism expert Timothy Furnish] said, has gone through several phases: hijacked airlines in the 1970s and 1980s, car and suicide bombs in the 1980s and 1990s. But the “shock value” of each inevitably wore off, giving way to something new “to maximize shock and press reaction upon which they thrive,” he wrote. “What once garnered days of commentary now generates only hours. Decapitation has become the latest fashion. In many ways, it sends terrorism back to the future. Unlike hijackings and car bombs, ritual beheading has a long precedent in Islamic theology and history.”

But “increasingly,” Furnish wrote, “Islamist groups conflate ‘unbelievers,’ ‘combatants,’ and prisoners of war, which, coupled with their claim to Islamic legitimacy, provides them with a license to decapitate.”

This license is one of ISIS’s most salient traits. Twitter is awash with images of its decapitations and worse. The result: Fear has become a potent ISIS weapon, according to this Amnesty International report called “Rule of Fear.”